Apparatus for control of hard water scale deposition in cooling systems

ABSTRACT

Disclosed is a method and apparatus for preventing or minimizing scale deposition from hard water onto heat transfer surfaces in atmospheric cooling towers, such as those used in air conditioning and refrigeration systems, where evaporative cooling of water provides the necessary heat sink to the atmosphere. An electrical/mechanical control system responsive to changes in density of the water in the evaporative cooling system operates to permit discharge of water containing dissolved and suspended solids from the system when its density reaches a predetermined maximum value and to discontinue the discharge of cooling system water when its density reaches a predetermined minimum value. Water drained from the cooling system is replaced by fresh water, preferably by water containing chemical additives to increase solubility of hard water minerals in the cooling water.

This application is a division of our copending U.S. patent applicationSer. No. 495,351 filed Mar. 19, 1990 U.S. Pat. No. 5,013,488.

BACKGROUND OF THE INVENTION

This invention relates to a method and means for preventing orminimizing the deposition of scale from hard water on water wet surfacesin evaporative cooling systems.

In the operation of an evaporative cooling system, water is lost fromthe system by evaporation which helps to cool the remaining water. Aswater is lost by evaporation, make-up water is added to maintain therequired water inventory in the system. At the same time, however,dissolved minerals are left behind by the evaporating water so that asmore and more water is lost by evaporation, the concentration ofdissolved solids in the remaining water grows greater and greater.Eventually, the concentration of dissolved solids reaches and exceedsthe saturation level with the result that solids begin to precipitateout of the water solution, and deposit as scale on the heat exchangesurfaces associated with the evaporative cooling system. Scaling on heattransfer surfaces decreases the rate of heat transfer. The loss of heattransfer efficiency leads not only to increased operating costs but alsoto deterioration of equipment.

A common method of preventing the build-up of dissolved minerals in thewater of an evaporative cooling system involves the addition ofchemicals, e.g. ethylenediaminetetraacetic acid, to maintain the scaleforming solids in solution, periodically draining a portion of the watercontaining dissolved scale and adding fresh make-up water. Manyproposals for preventing scale deposition have been made. A leadingmethod involves chemical treatment of the water, measuring theconductivity of the water in the system, draining water containingdissolved solids from the system in response to the conductivitymeasurement when the preset limit on dissolved solids is reached andsupplying fresh water and chemicals to the system. Even with such amethod, there is a need for one which does not require as much drainageof water from the system and therefore lower chemicals consumption. Asdescribed in greater detail hereafter, this invention provides areliable method of limiting the build-up of dissolved minerals in thewater of an evaporative cooling system to a predetermined level and atthe same time restricting the drainage of water. The method is automatedby a reliable but comparatively inexpensive means for carrying out thecontrol method.

SUMMARY OF THE INVENTION

In accordance with this invention, the method of control of the build-upof solids in the water of an evaporative cooling system is based on adensity measuring device.

The density measurement device of this invention comprises a samplingvessel or chamber connected to the cooling system and arranged for theflow therethrough of some of the water and associated suspended solidscirculating through the system, a hydrometer in the chamber, meansresponsive to the float position, i.e. when the float rises apredetermined distance relative to the surface of the water due toincreasing density of the system water, and valve operating means foropening a valve in a water discharge line to discharge water from thesystem.

A level control valve in the make-up water line admits water andchemicals into the system to replenish that which is discarded as wellas that lost by evaporation. When this valve in the make-up water linesopens, the flow of water through a flowmeter associated with thechemical feed device promptly activates an associated pump to inject apredetermined proportion of the chemical solution into fresh waterentering cooling system. The relative proportions of chemical solutionand water is based on the hardness of the available water supply. Therate of injection of chemicals is based on the flow rate of the make- 0up water through the flowmeter. A practical device for dispensingchemicals into a stream of water is marketed by Liquid Metronics Inc.,an established supplier of water treatment equipment. This operation ofdischarging water containing a high solids content from the system andadding fresh makeup water and chemical additives continuous until thefloat in the sampling chamber reaches a predetermined lower position atwhich time the valve operating means acts to close the valve in thewater discharge line. The cycle is repeated as often as necessary duringoperation of the cooling system.

Many such chemicals and chemical compositions for water treatment aremarketed. However, the preferred chemical composition for use in thisinvention is the composition marketed under the registered trademarkScalesolve by Descale-It Products Company, of Tucson, Ariz., describedin U.S. Pat. No. 4,595,517. It has been found that the composition hasthe capability of emulsifying undissolved scale and holding anappreciable amount of precipitated water minerals in suspension withoutthe accumulation of scale on heat transfer surfaces.. With such chemicalcompositions which have the capability of maintaining scale formingprecipitates in suspension, the system water can be permitted to havesuch a build-up of hardness and suspended minerals that the density ofthe water can go as high as about 1.04 grams per milliliter (g/ml)without substantial deposition of scale on heat transfer surfaces Innormal operation, it is preferred to maintain the density in the rangeof 1.004 to 1.02 g/ml.

An economically important feature of the invention is that it isunnecessary to discard large amounts of chemically treated system waterin order to avoid scaling in heat exchangers. Many existing coolingsystems have a preset rate of continuous drainage which is costly bothin the wasteful consumption of water and of the chemical added to it toprevent scale formation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, the further descriptionthereof will refer to the accompanying drawings of which:

FIG. 1 is a diagrammatic elevational view of an evaporative coolingsystem illustrating a preferred embodiment of the invention;

FIG. 2 is a detailed cross sectional view of the hydrometer floatchamber of FIG. 1 illustrating a preferred method of control of thedensity of water in an evaporative cooling system.

FIG. 3 is a schematic diagram of electrical and electronic circuitryillustrating a preferred embodiment cf apparatus forming part of thisinvention.

DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIG. 1, illustrating one specific preferred embodimentof apparatus sutable for carrying out the method of this invention,reference numeral 10 designates an atmospheric cooling tower of anevaporative cooling system. Inasmuch as the fans, pumps, heat exchangersand piping conventional in such systems are not essential to thedescription of this invention, they have been omitted in the interest ofclarity. Sump 11 of tower 10 is connected by pipe 12 to pump 13 whichcontinuously recirculates water from the sump 11 via pipe 14 to the topof tower 10. A sample line 15 provided with needle valve 16 serves topass a slip stream of water from pipe 14 to sampling chamber 17 whichhas overflow pipe 18 draining into tower 10. A hydrometer 19 floats inthe pool of water in the lower part of chamber 17. Light emitting diodes20 and 20' opposite a pair of detectors 21 and 21' are positioned in theupper portion of chamber 17. When the water passing through chamber 17has a density equal to or exceeding a predetermined value, e.g., 1.02g/ml, hydrometer 19 rises in the water pool and the tip of its spindle19A will interrupt the light beam directed from light source 20 to lightdetector 21 both of which are connected by wires 22 and 23,respectively, to circuit board 24. Similarly, light source 20' and lightdetector 21' are connected to circuit board 24 as described in moredetail hereinafter by wires 25 and 26.

When spindle 19A of hydrometer 19 rises and intersects the light beam ofemitter 20' preventing it from impinging on detector 21' because thewater in chamber 17 has a density equal to or exceeding the selectedmaximum, the circuitry of board 24 sends electrical power via wires 27,27' and 27" to normally closed solenoid valve 28 in drain line 29causing valve 28 to open and thus discard water from the cooling system.

As water is discarded from the system through ppe 29, the liquid levelin sump 11 of tower 10 falls and causes floatcontrolled valve 30 to openso that fresh make-up water containing chemical additives from supplypipe 31 flows into sump 11. Flowmeter-pulser 32, responsive to the rateof flow of mak.-e up water through pipe 31 sends electrical pulses to adiaphragm type liquid metering pump 35 which delivers precise amounts ofchemical additive from chemical supply tank 36 through pipe 37 to pump35 and from pump 35 through pipe 38 to make-up water supply line 31.

Hence, when valve 30 opens and make-up water flows through pipe 31 anelectrical sgnal from flowmeter-pulser 32 is transmltted to andactivates pump 35 so that chemical from tank 36 is injected via line 38into the make-up water flowing through pipe 31. The proportion ofchemical to make-up water is preset and controlled by flowmeter-pulser32. As previously mentioned, the selected proportioning is based on thehardness of fresh make-up water and the effectiveness of the injectedchemical or chemical composition.

During the simultaneous discharge of system water from pipe 29 andintroduction of fresh water through pipe 31, the density of the systemwater continuously recirculated through pipes 12, 14, 15, 18 graduallydecreases and ultimately drops to or below a preselected value. Ashydrometer 19 moves downward in chamber 17 responsive to the decrease indensity of the water in the system, it reaches a point where spindle 19Ano longer obstructs the light beam from source 20 to detector 21. Whendetector 21 detects light from light source 20 it sends a signal to thecircuitry of circuit board 24, Which in turn acts to cause solenoidvalve 28 to close. When the discharge of system water is stopped, theflow of make-up water to sump 11 soon raises the water level thereinsufficiently to cause level control valve 30 to close stopping the flowof water through flowmeter-pulser 32 and the flow of chemical additivethrough lines 37 and 38.

With valve 28 closed, the flowmeter-pulser 32 and chemical feed device35 are in stand-by condition ready to become operative when the floatvalve 30 calls for make-up water addition due to evaporation of water inthe system. When the density of the system water passing throughsampling chamber 17 causes spindle 19A to interrupt the light beamdirected by source 20 toward detector 21, solenoid valve 28 is opened todischarge system water and the consequent drop of water level in sump 11opens level control valve 30 to replenish the water discarded from thesystem, thus completing the cycle.

Sampling chamber 17 is shown diagrammatically in FIG. 1 in its basicform. FIG. 2 shows details of chamber 17 with additional elements thatmaintain hydrometer 19 in a central position and that minimize watercurrent impingement on hydrometer 19.

With references to FIG. 2, vessel 17 is desirably a clear plastic tubecapped at its opposite ends by plastic disks 41 and 42. Another clearplastic tube 43 of smaller diameter than vessel 17 is heldconcentrically within tube 17 by disk 45. The lower portion of innertube 43 has several small openings or perforations 47 which permit waterentering outer tube 17 through pipe 15 to pass into and out of tube 43on its way to drain pipe 18. Tube 43 with perforations 47 serves tominimize the impingement on hydrometer 19 of the water-current flowingthrough chamber 17. The upper portion of inner tube 43 contains severalvertically spaced horizontal plastic disks 44 each with a centralopening 48 of slightly larger diameter than the diameter of spindle 19Aof hydrometer 19. Thus, disks 44 act to keep spindle 19A substantiallyaligned with the axis of chamber 17 and tube 43. The central openings 48are only large enough to allow spindle 19A to move freely up and down asthe density of the water passing through chamber 17 varies. The topportion of inner tube 43 is provided with light sources 20 and 20' andlight detectors 21 and 21' mounted therein in diametrically oppositepositions. Wiring 22, 23, 24, 25, associated with light sources 20 and20' and light detectors 21 and 21' is illustrated as exiting vessel 17through top disk 42 but may, if desired, pass through the wall of tube17.

The inner tube 43 is also provided near its lower end with a pair ofdisks 49 and 49', each having an openng at its center larger than thetip of hydrometer 19. A screen 50 sandwiched between plates 49 and 49'extends over the central opening and is held in place by the plates. Thescreen 50 limits the downward travel of hydrometer 19 in the event thatthe water level in sampling chamber 17 falls below the level of outletline 18.

A preferred specific embodiment of an electrical circuit capable ofperforming the above described functions is illustrated in FIG. 3. Withreference to this figure, infrared light emitting diodes 20 and 20' arepaired with infrared detectors 21 and 21' in conventional circuitswherein 5 V is a 5 volt power source and resistors R₁ limit the currentpassing through the infrared light emitting diodes. When infrared lightis received by the infrared detector 21 or 21' its resistance decreasesallowing a large increase in current to flow from its 5 volt powersource 5 V through the detector, which, in turn, causes a large increasein voltage drop across its resistor R₂. For example, in this particularcircuit, when R₂ has a value of 1000 ohms the voltage drop varies fromnearly 0 when the beam is locked by stem 19' of the hydrometer 19 toabout 3.5 volts when the infrared beam is not blocked.

The voltage drop across resistor R₂ supplies the desired operationalcontrol signal for operation of the control system via operationalamplifiers 60 and 60' which operate as noninverting amplifiers. The gainin voltage R₄ /R₃, increases the voltage in line 61 by an amountsufficient, e.g. 12 to 15 volts, to ensure an "on" position at the NORgates as described hereinafter. A NOR gate 62 in line 61 inverts thepolarity of the signal in line 61. The two signals in lines 61 and 61'are transmitted to NOR gates 63 and 63' arranged as shown in thedrawings to function as an RS flip-flop, a simple logic circuit ofconventional design well known in the art.

In the particular circuit illustrated, the output from the flip-flopcircuit 63, 63' passes through line 64 and through two operationalamplifiers 66 and 68 which operate as inverting amplifiers in series toan NPN, 10-50 hFE power transistor 70. Power transistor 70 acts as aswitch to permit current to flow from a 15 volt power source 15 Vthrough the solenoid winding 71 of a double pole, double throw relayswtch 72 of conventional design which activates solenoid valve 28 ofFIG. 1. The +12 volt supply to the base lead of transistor 70 producesapproximately 11.4 volts at 90 milliamperes at the emitter lead toenergize the solenoid winding 71 of relay switch 72 which, in turn,allows current to flow through lines 27, 27' and 27" energizing thesolenoid of valve 28 opening the valve. When no signal is supplied tothe transistor 70 from amplifier 68, no current flows from voltagesource 15 V which then allows valve 28 of FIG. 1 to return to itsnormally closed position.

In the specific embodiment of the control circuit illustrated in FIG. 3,the following values of resistors R₁ through R₈ are listed in Table I.

                  TABLE I                                                         ______________________________________                                        Resistor     Value                                                            ______________________________________                                        R.sub.1      430          ohms                                                R.sub.2      1000         ohms                                                R.sub.3      1.5          megohms                                             R.sub.4      6.8          megohms                                             R.sub.5      1.6          megohms                                             R.sub.6      2.0          megohms                                             R.sub.7      1.6          megohms                                             R.sub.8      1000         ohms                                                ______________________________________                                    

We claim:
 1. Apparatus responsive to a first variable representingchanges in conditions of a liquid in a heat transfer system to controlcompensating conditiosn in a related second variable which comprisesfirst and second detector means responsive to changes in said firstvariable for determining first and second predetermined limitingconditions thereof, said means comprising first and second output meansproviding electrical signals indicative of said first and secondconditions respectively, a first amplifier having an input coupled toreceive the electrical signal from said first detector output, a secondamplifier having an input coupled to receive the electrical signal fromsaid second detector output, means comprising an inverter having aninput coupled to the output of said first amplifier for providing anoutput signal inverted in polarity with respect to its input signal,inverter and the output of said second amplifier, output means forderiving an output signal from said RS flip-flop circuit including aflow control means controlling addition of liquid to said system andremoval of liquid therefrom as said related second variable. 2.Apparatus according to claim 1 wherein said first and second conditionscorrespond to the density of the liquid in the heat transfer system. 3.Apparatus according to claim 1 wherein said first variable is thedensity of a solution of solids in said liquid and said related secondvariable is the rate of dilution of said solution by addition of freshliquid solvent thereto.
 4. Apparatus according to claim 3 wherein thedetector means comprise first and second electro-optical light detectorsresponsive to the position of a hydrometer float in a representativeportion of said solution.
 5. Apparatus according to claim 4 wherein thedetector means are responsive to light, the path of which is interruptedby said float when one predetermined limiting condition is reached anduninterrupted when the other limiting condition is reached.
 6. Apparatusaccording to claim 5 wherein the light path to both detectors isinterrupted by said float when the maximum predetermined limit ofsolution density is reached and neither is interrupted when the minimumpredetermined limit of solution density is reached.
 7. Apparatusaccording to claim 1 wherein the detector means comprises anelectro-optical light detector responsive to changes in said firstvariable.